1. Field of the Invention
The present invention is related to a cellular communication system. In particular, the present invention is related to a cellular communication system that enables an improved interference monitoring.
2. Description of the Related Art
Interference is one of the issues in a cellular communication system. The interference degrades the service quality of the cellular communication system. Especially, interference from another independent cellular communication system often causes serious influence on the service quality. The influence of the interference should be suppressed to provide superior service quality.
Cellular communication systems are often designed so as to suppress the influence of the interference. Hamabe discloses a cellular communication system for suppressing the influence of the interference in Japanese Patent Gazette (JP-B 2661533). A cell station in the disclosed cellular communication system includes a direct wave direction determining unit for determining the incoming direction of a direct wave from a personal station, and an interfering wave direction determining unit for determining the incoming direction of an interfering wave. The cell station selects one of channels such that the angle between the incoming directions of the interfering wave and the direct wave is close to 180°. The selected channel is allocated to the communication service. This results in that the personal station receives the direct wave and the interfering wave in the same direction, and thus improves the intensity ratio of the direct wave and the interfering wave. Accordingly, the influence of the interference is reduced in the cellular communication system.
Hamabe also discloses another cellular communication technique for avoiding the interference in Japanese Open Laid Patent Application (JP-A-Heisei 11-341555). The cellular communication technique aims to reduce interference between a plurality of cellular systems, which are denoted by A and B, whose service areas are overlapped each other. The cellular system A is allowed to use a plurality of carrier frequencies, one of which is adjacent to a carrier frequency which cellular communication systems B is allowed to use, and others of which are not adjacent thereto. The cellular system A uses the carrier frequency not adjacent to the carrier frequency used by the cellular system B when received power of a pilot channel sent by a cell station of the cellular system B is higher than that of a pilot channel sent by a cell station of the cellular system A.
Kanei discloses still another cellular communication system in Japanese Patent Gazette (JP-B 2699854), which decreases a number of frequencies at which carrier-to-interference ratios are measured while the channel allocation. In the cellular communication system, communication channels are classified into first and second channel groups. The communication channels classified into the first channel group are given the same priority, while the other communication channels classified into the second channel group are respectively given a specific priorities. The priorities given to the communication channels classified into the second channel group are modified in response to the interference level of the communication channels. Autonomous Reuse Partitioning is utilized for allocating the communication channels. When a mobile station request a communication, carrier to-interference ratios (CIRs) are detected in the order of the priority. When a detected CIR is firstly satisfied with a predetermined CIR condition, the communication channel corresponding to the detected CIR is allocated to the mobile station.
Fukui et al. disclose still another cellular communication system in Japanese Open Laid Patent Application (JP-A-Heisei 8-331637). In the cellular communication system, a mobile station measures carrier-to-interference ratios (CIRs) to store them therein. The mobile station makes prospect of the CIR on the basis of the stored CIRs. When the mobile station requests communication, the mobile station selects one of the communication channels on the basis of the prospect of the CIR.
Ogura et al. disclose still another cellular communication system for improving frequency utilization efficiency in Japanese Patent Gazette (JP-B 3029576). In the cellular communication system, cell stations respectively transmit interference measuring channels. A mobile station measures the electric field intensities of received signals transmitted on the interference measuring channels. The distances between the mobile station and the cell stations are estimated on the basis of the electric field intensities. The allocation order of the communication channels is modified on the basis of the estimated distances to improving frequency utilization efficiency.
To design a cellular communication system so as to suppress the influence of the interference, a detailed monitoring of the interference profile is helpful.
Conventional communication protocols such as a time division multiple access (TDMA) system protocol and a code division multiple access (CDMA) system protocol, execute a detection of the interference. The communication system allows a mobile station and a cell station to communicate after ensuring that the interference is below the predetermined level.
However, the interference detection executed by the usual communication protocol is not satisfactory for designing the cellular communication system. The detection is executed only at timings predetermined by the communication protocol. This means that the interference is detected only at a predetermined interval, that is, the interference is not detected while user messages are being communicated.
To obtain a detailed interference profile, interference measurement apparatuses are often used. For obtaining a detailed interference profile, an interference measurement apparatus is set beside the cell stations, and the interference in the cellular communication system is monitored with the interference measurement apparatus for a desired duration.
An improved technique is desired which enables acquisition of a detailed profile of interference in a cellular communication system.